1. Field of the Invention
The present invention relates to a display driver, a display device having the display driver, and a portable electronic apparatus having the display device. More specifically, the present invention relates to a display driver having a memory cell which is constructed by a field effect transistor and includes a memory functional unit having a function for retaining charges or polarizations, a display device including the display driver, and a portable electronic apparatus including the display.
2. Description of the Related Art
In a display using a liquid crystal, an organic EL or the like, various set values are preset. For example, a gamma value as an example of the set value is used to correct the relation between an inputted tone signal and brightness on the display. In a conventional display device, the set values are stored in a nonvolatile memory such as an EEPROM attached to a display driver for driving the display device. Concretely, a chip on which an EEPROM is mounted is externally attached to the display driver for driving the display device.
A flash memory as the mode of the EEPROM will be described below as an example. FIG. 24 is a schematic sectional view showing an example of the flash memory cell. Shown in FIG. 24 are a semiconductor substrate 901, a floating gate 902, a word line (control gate) 903, a diffusion layer source line 904, a diffusion layer bit line 905, a device isolation region 906, and an insulating film 907.
The flash memory cell stores memory information in accordance with an amount of charges in the floating gate. In a memory cell array constructed by arranging memory cells, an operation of rewriting/reading for a desired one of the memory cells can be performed by selecting a specific word line and a specific bit line and applying a predetermined voltage to them.
FIG. 25 is a graph schematically showing a drain current (Id)-gate voltage (Vg) characteristic when the amount of charges in the floating gate in the flash memory changes. When the amount of negative charges in the floating gate increases, the threshold increases and the Id-Vg curve shifts almost in parallel in the Vg increasing direction (see, for example, Japanese Unexamined Patent Publication No. Hei 05-304277 (1993)).
However, since the EEPROM has the floating gate, two polysilicon layers for forming the floating gate and control gate have to be patterned, and a process is complicated. It disturbs reduction in the cost of a display driver having the EEPROM. Reduction in the cost of the display driver is demanded, and it is also desired to mount a nonvolatile memory which can be manufactured at low cost in place of the EEPROM.